Single-wall carbon nanotubes (SWNT) are fullerenes of closed-cage carbon molecules typically arranged in hexagons and pentagons. Commonly known as “buckytubes,” these cylindrical carbon structures have extraordinary properties, including high electrical and thermal conductivity, as well as high strength and stiffness. (See B. I. Yakobson and R. E. Smalley, American Scientist, Vol. 85, July–August, 1997, pp. 324–337.)
With an intrinsic strength estimated to be on the order of 100 times that of steel, single-wall carbon nanotubes are a possible strengthening reinforcement in composite materials. The intrinsic electronic properties of single-wall carbon nanotubes also make them electrical conductors and useful in applications involving field emission devices, such as flat-panel displays, and in polymers used for radiofrequency interference and electromagnetic shielding that require electrical conductance properties. In other applications involving electrical conduction, single-wall carbon nanotubes and ropes of single-wall carbon nanotubes are useful in electrically conductive coatings, polymers, paints, solders, fibers, electrical circuitry, and electronic devices, including batteries, capacitors, transistors, memory elements, current control elements, switches and electrical connectors in micro-devices such as integrated circuits and semiconductor chips used in computers. The nanotubes are also useful as antennas at optical frequencies as constituents of non-linear optical devices and as probes for scanning probe microscopy such as are used in scanning tunneling microscopes (STM) and atomic force microscopes (AFM). Their exceptional thermal conductivity properties render single-wall carbon nanotubes useful in composites, coatings, pastes, paints and other materials where heat transfer is a desired property. In composite materials, aligned single-wall carbon nanotubes can provide enhanced electrical, mechanical, optical, and/or thermal properties. Single-wall carbon nanotubes can be used as replacement for, or in conjunction with, carbon black in tires for motor vehicles, and as elements of composite materials to elicit specific physical, chemical or mechanical properties in those materials, such as electrical and/or thermal conductivity, chemical inertness, mechanical toughness, etc. The nanotubes themselves and materials and structures comprising carbon nanotubes are also useful as supports for catalysts in chemical processes, such as hydrogenation, polymerization and cracking, and in devices such as fuel cells.
To capture the exceptional properties of single-wall carbon nanotubes, numerous attempts have been made to incorporate the nanotubes into other materials, such as polymers, ceramics, metals and materials of manufacture. However, one of the problems encountered in making composite blends is the difficulty in dispersing single-wall carbon nanotubes. With a better dispersion of the nanotubes, more of the nanotube properties could be imparted to the composite medium at a lower nanotube loading.
The largest complication in dispersing single-wall carbon nanotubes is their propensity to tightly self-associate with each other. When single-wall carbon nanotubes come in close contact with each other, they tend to become tightly bound by van der Waals forces, which act to hold the nanotubes tightly together as “ropes” of aligned bundles of a few to many hundreds of nanotubes. Besides this ordered roping alignment, there is also significant disordered entanglement when many of the single-wall carbon nanotubes and ropes of single-wall carbon nanotubes contact each other randomly during synthesis, external compression and/or subsequent purification. These randomly oriented, entangled mats of individual single-wall carbon nanotubes and ropes of single-wall carbon nanotubes are very difficult to disperse into other materials, such as polymers, either as individual single-wall carbon nanotubes or ropes of single-wall carbon nanotubes. The compression and matting is especially problematic after purification processes involving liquid-phase treatments, such as described in Chiang, et. al., “Purification and Characterization of Single-Wall Carbon Nanotubes,” J. Phys. Chem. B, 105, 1157–1161, (2001). In this procedure and other wet methods, the single-wall carbon nanotubes are wetted with water or some other solvent, either in the chemical purification or as part of the filtering and washing. Subsequent drying by evaporation causes the single-wall carbon nanotubes to more closely associate and remain tightly associated through van der Waals interactions. With evaporation drying, the bulk density of the single-wall carbon nanotubes increases more than an order of magnitude over the initial raw material whose bulk density is of the order of 0.01 g/cc. Densification occurs because capillary forces promote a collapse of the space between the ropes of single-wall carbon nanotubes that exist in the original sample. A denser single-wall carbon nanotube product complicates the formation of a substantially uniform dispersion of single-wall carbon nanotubes in applications where dispersal of the individual single-wall carbon nanotube segments and ropes is desirable or required. Redispersing the individual single-wall carbon nanotubes or single-wall carbon nanotube ropes after they are in the denser matted form is difficult and problematic. Further processing to achieve redispersion may not only affect the nanotube properties, but also increases the cost of composites and final products due to higher labor and equipment requirements.
A related complication in dispersing single-wall carbon nanotubes is that due to their chemical composition and structure, the nanotubes are generally quite insoluble in liquids and other media. The nanotubes would generally tend to self-associate with each other through van der Waals interactions rather than disperse in other media.
The ability to disperse single-wall carbon nanotubes remains one of the largest barriers in realizing the full potential of single-wall carbon nanotubes in various applications. Besides the challenge of dispersing single-wall carbon nanotubes, even when dispersed, the single-wall carbon nanotubes and ropes of single wall carbon nanotubes may not provide the optimum configuration to achieve the full potential of the strength and properties of the nanotubes unless they are aligned. Controlled alignment of single-wall carbon nanotubes from surfactant-assisted suspensions and fabrication of macroscopic forms of single-wall carbon nanotubes such as fibers or shear-aligned aggregates face the inherent limitations of the single-wall carbon nanotube-surfactant system. Since the van der Waals forces between the single-wall carbon nanotubes and ropes of single-wall carbon nanotubes are larger than the weak electrostatic repulsions arising from the adsorbed surfactant molecules, the single-wall carbon nanotube solutions are generally very low in concentration and impractical for many applications. Although oriented single-wall carbon nanotube fibers could be prepared with surfactant dispersions by shear flow-induced alignment in a co-flowing stream of polymer solution, the single-wall carbon nanotube concentrations attainable in a sodium dodecyl benzene sulfonate/single-wall carbon nanotube/water system are generally too low (i.e., less than about 1 wt %) to achieve coordinated single-wall carbon nanotube alignment.
Some methods to disperse single-wall carbon nanotubes have focused on overcoming the van der Waals forces which hold the nanotubes in intimate contact. One chemical approach to separating the nanotubes and making them more soluble includes functionalization with solubilizing moieties, either on the ends and/or the sides of the nanotubes. See “Carbon Fibers Formed from Single-Wall Carbon Nanotubes,” International Pat. Publ. WO 98/39250 published Sep. 11, 1998, and “Chemical Derivatization of Single-Wall Carbon Nanotubes to Facilitate Solvation Thereof, and Use of Derivatized Nanotubes,” International Pat. Publ. WO 00/17101, published Mar. 30, 2000, both of which are incorporated by reference herein in their entirety. Another way of dispersing single-wall carbon nanotubes is by introducing an intercalating species that will separate the nanotubes using a physio-chemical approach. Oleum, a well-known superacid, has been used as an intercalating species so as to suspend and disperse single-wall carbon nanotubes and make large “super ropes” of aligned nanotubes. See “Macroscopic Ordered Assembly of Carbon Nanotubes,” International Pat. Publ. WO 01/30694 A1, published May 3, 2001, incorporated by reference herein in its entirety. Physical methods for inducing separation of the nanotubes have included sonication and other means of intensive mixing. However, these aggressive techniques can induce damage, shear and breakage in the nanotubes, and, thereby, compromise the desired nanotube properties for the intended application.
Wrapping single-wall carbon nanotubes with amphiphilic polymers has also been shown as a means to overcome van der Waals forces between single-wall carbon nanotubes. See “Polymer-Wrapped Single Wall Carbon Nanotubes,” International Pat. Publ. WO 02/16257 published Feb. 28, 2002, incorporated by reference herein in its entirety. Although polymer wrapping of the nanotube enables the dispersion of single-wall carbon nanotubes in water and other solvents, higher concentrations of nanotubes dispersed in other media over a broad range of temperature conditions are often desired.
Thus, there is a need for a form of single-wall carbon nanotubes in which the nanotubes are aligned and can be dispersed in other media, such as polymers, ceramics, metals and other media used in manufacture. There is also a need to be able to redisperse the aligned aggregate into individual nanotubes or smaller aggregates of single-wall carbon nanotubes. Likewise, there is a need for composites comprising dispersed, highly aligned single-wall carbon nanotubes.